1. Field of the Invention
This invention pertains to organosiloxane/oxyalkylene copolymers. More particularly, this invention pertains to polyorganosiloxanes containing pendant oxyalkylene units that are terminated with an acrylate group. The copolymers are curable by ultraviolet or electron beam radiation and are particularly useful as electrolytes in conjunction with solubilized, ionizable lithium salts.
2. Description of the Prior Art
Organosiloxane/oxyalkylene copolymers containing divalent siloxane units corresponding to the general formula RGSiO are known in the art. In this formula R typically represents a monovalent hydrocarbon radical or a substituted monovalent hydrocarbon radical and G represents a sequence of at least two oxyalkylene groups such as oxyethylene or oxypropylene that are bonded to the silicon atom by means of an alkylene radical. The sequence can be terminated with a hydroxyl group, an alkoxide group or a group of the formula --O(O)CR', where R' represents a monovalent hydrocarbon radical that is free of ethylenic unsaturation.
U.S. Pat. No. 3,703,489, which issued to Morehouse on Nov. 21, 1972 describes silane/oxyalkylene block copolymers wherein from 15 to 60 weight percent of the oxyalkylene units are oxyethylene and the remainder are oxypropylene. The sequences of oxyalkylene units are terminated with an alkoxy group containing from 1 to 10 carbon atoms.
The sequence of oxyethylene units in the siloxane/oxyethylene copolymers disclosed in U.S. Pat. No. 3,560,544, which issued to Haluska on Feb. 2, 1971 are bonded to silicon through an alkylene group and are terminated by groups of the formula --OC(O)R[C(O)OA].sub.x, where A represents an amine group, an alkali metal or an alkaline earth metal, R represents a divalent or a trivalent hydrocarbon radical containing from 2 to 6 carbon atoms and x is 1 or 2.
U.S. Pat. No. 3,957,843, which issued to Bennett on May 18, 1976, discloses siloxane/oxyalkylene copolymers wherein A of the foregoing formula is hydrogen, a monovalent hydrocarbon radical free of ethylenic unsaturation, --OCNHR', --OCNH2, --OCR', or --OCOR', where R' represents a monovalent hydrocarbon radical that is free of aliphatic unsaturation.
Polyorganosiloxanes containing acrylate functional pendant groups of the formula --R"C--O(O)CCH.dbd.CH.sub.2 bonded to at least a portion of the silicon atoms are described in U.S. Pat. No. 3,878,263, which issued to Martin on July 10, 1972. In this instance the acrylate group is bonded to silicon through a divalent hydrocarbon radical that can optionally contain an ether linkage. A sequence of oxyalkylene units is not mentioned as a possible candidate for the R" group.
The absence of acrylate or other ethylenically unsaturated terminal groups on many prior art siloxane/oxyalkylene copolymers is understandable, based on the intended use of these copolymers. All of the references cited hereinabove that disclose siloxane/oxyalkylene copolymers teach using these copolymers as surfactants for the preparation of polyurethane foams. These surfactants are intended to facilitate foam formation, and typically do not require the presence of ethylenically unsaturated hydrocarbon radicals.
The prior art pertaining to the use of organosiloxane/oxyalklyene copolymers as surfactants go so far as to specifically exclude the presence of ethylenically unsaturated carbon-to-carbon bonds, usually to avoid undesired side reactions during preparation of these copolymers. The copolymers are typically prepared by reacting a monoallyl ether of the desired oxyalkylene polymer with a polyorganohydrogensiloxane containing an average of at least one silicon-bonded hydrogen atom per molecule. The allyl or other ethylenically unsaturated hydrocarbon radical reacts with the silicon-bonded hydrogen atoms, and appear in the final product as an alkylene radicals.
Another known use for liquid polydiorganosiloxanes containing pendant polyoxyalkylene units is as electrolytes in solid state batteries containing ionizable lithium compounds. An article by Hall et al. [Polymer Communications, 27 (4), 98-100 (1986)] discloses copolymers prepared by the base catalyzed reaction between a polymethylhydrogensiloxane and ethylene glycol oligomers terminated on one end by a methoxy group and on the other end by a hydroxyl group. The reaction product could be crosslinked by heating, and the conductivity was found to be inversely proportional to the degree of crosslinking.
For many applications, including solid state batteries, it is preferred if not a requirement that the electrolyte be a solid material. This can be achieved by curing the polymeric electrolyte either prior to or following addition of the ionizable lithium salt.
An article by Fish et al. [Makromol. Chem, Rapid Comm. 7, 115-120 (1986)] reports conductivity data for reaction products of a polymethylhydrogensiloxane and a methoxy-terminated polyethylene oxide. Lithium perchlorate was added to the copolymer before it was cured by heating in the presence of benzoyl peroxide.
An article by Bouridah et al. [Solid State Ionics, 15 (1985), 233-240] describes electrolytes prepared by adding lithium perchlorate to a dimethylsiloxane/grafted ethylene oxide copolymer of the formula Me.sub.3 SiO(Me.sub.2 SiO).sub.x (MePEOSiO).sub.y SiCH.sub.3 where Me represents methyl, the value of x is about 56, the average value of y is about 16 and PEO is --(CH.sub.2 CH.sub.2 O).sub.22 --. The electrolyte is cured by reacting it with an isocyanate. The problem with this method for crosslinking the polymer is that urethane groups formed during the crosslinking reaction contain nitrogen-bonded active hydrogen atoms that can interfere with the electrochemical reactions occurring in batteries.
Japanese published application no. 217263/85 discloses the addition of lithium perchlorate to a cured polymer having repeating units of the formula ##STR1## where Me represents methyl and the value for n not specified but is equivalent to a liquid polymer. The polymer is swollen using acetone prior to addition of the lithium salt.
Crosslinked polymers are typically undesirable as starting materials for preparing electrolytes because it is difficult to incorporate large amounts of ionizable salts into these materials even after the polymers are swollen using organic liquids such as acetone.
An article by Nagaka et al. [J. Poly. Sci., Polymer Letters, 22 (12), 659-63, 1984] reports high ionic conductivity for uncured polymers having repeating units of the formula disclosed in the aforementioned Japanese published application and containing a solubilized lithium salt.
The present inventors have found that organosiloxane/oxyethylene copolymers of the prior art containing solubilized lithium salts are often difficult to cure using organic peroxides or a hydrosilation reaction between silicon-bonded hydrogen atoms and lower alkenyl radicals such as vinyl. One aspect of the present invention resides in a class of organosiloxane/oxyethylene copolymers that do not have this disadvantage.